The present invention relates to ducted fan vehicles, and particularly to such vehicles useful as VTOL (Vertical Take-Off and Landing) aircraft.
Many different types of VTOL aircraft have been proposed where the weight of the vehicle in hover is carried directly by rotors or propellers, with the axis of rotation perpendicular to the ground. One well known vehicle of this type is the conventional helicopter which includes a large rotor mounted above the vehicle fuselage. Other types of vehicles rely on propellers that are installed inside circular cavities, shrouds, ducts or other types of nacelle, where the propeller or rotor is not exposed, and where the flow of air takes place inside the circular duct. Most ducts have uniform cross-sections with the exit area (usually at the bottom of the duct when the vehicle is hovering) being similar to that of the inlet area (at the top of the duct). Some ducts, however, are slightly divergent, having an exit area that is larger than the inlet area, as this was found to increase efficiency and reduce the power required per unit of lift for a given inlet diameter. Some ducts have a wide inlet lip in order to augment the thrust obtained, especially in hover.
VTOL vehicles are usually more challenging than fixed wing aircraft in terms of stability and control. The main difficulty rises from the fact that, contrary to fixed wing aircraft which accelerate on the ground until enough airspeed is achieved on their flight surfaces, VTOL vehicles hover with sometimes zero forward airspeed. For these vehicles, the control relies on utilizing the rotors or propellers themselves, or the flow of air that they produce to create control forces and moments and forces around the vehicle's center of gravity (CG).
One method, which is very common in helicopters, is to mechanically change, by command from the pilot, the pitch of the rotating rotor blades both collectively and cyclically, and to modify the main thrust as well as moments and/or inclination of the propeller's thrust line that the propeller or rotor exerts on the vehicle. Some VTOL vehicles using ducted or other propellers that are mounted inside the vehicle also employ this method of control. Some designers choose to change only the angle of all the blades using ducted or other propellers that are mounted inside the vehicle for this method of control. The angle of all the blades may be changed simultaneously (termed collective control) to avoid the added complexity of changing the angle of each blade individually (termed cyclic control). On vehicles using multiple fans which are relatively far from the CG, different collective control settings can be used on each fan to produce the desired control moments.
The disadvantage of using collective controls, and especially cyclic controls, lies in their added complexity, weight and cost. Therefore, a simple thrust unit that is also able to generate moments and side forces, while still retaining a simple rotor not needing cyclic blade pitch angle changes, has an advantage over the more complex solution. The main problem is usually the creation of rotational moments of sufficient magnitude required for control.
One traditional way of creating moments on ducted fans is to mount a discrete number of vanes at or slightly below the exit section of the duct. These vanes, which are immersed in the flow exiting the duct, can be deflected to create a side force. Since the vehicle's center of gravity is in most cases at a distance above these vanes, the side force on the vanes also creates a moment around the vehicle's CG.
However, one problem associated with vanes mounted at the exit of the duct in the usual arrangement as described above, is that even if these are able to create some moment in the desired direction, they cannot do so without creating at the same time a significant side force that has an unwanted secondary effect on the vehicle. For such vanes mounted below the vehicle's CG (which is the predominant case in practical VTOL vehicles), these side forces cause the vehicle to accelerate in directions which are usually counter-productive to the result desired through the generation of the moments by the same vanes, thereby limiting their usefulness on such vehicles.
The Chrysler VZ-6 VTOL flying car uses vanes on the exit side of the duct, together with a small number of very large wings mounted outside and above the duct inlet area.
However, in the VZ-6, the single wing and the discrete vanes were used solely for the purpose of creating a steady, constant forward propulsive force, and not for creating varying control moments as part of the stability and control system of the vehicle.
The Hornet unmanned vehicle developed by AD&D, also experimented with using either a single, movable large wing mounted outside and above the inlet, or, alternatively using a small number of vanes close to the inlet side. However these were fixed in angle and could not be moved in flight.
Another case that is sometimes seen is that of vanes installed radially from the center of the duct outwards, for the purpose of creating yawing moments (around the propeller's axis).